In the related art, various efforts have been made to increase a transmission capacity (hereinafter, referred to as a “system capacity” in some cases) in a communication system. For example, regarding the 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP LTE), which is a communication standard, discussion has been made on a technique for increasing the system capacity utilizing a femtocell in addition to a macrocell. A “cell” herein is defined based on a “communication area” and a “channel frequency” of one base station. The “communication area” may be the entire area where radio waves transmitted from the base station reach, or a divided area (what is called a sector) obtained by dividing the communication area. The “channel frequency” is a unit of a frequency used for communication by the base station, and defined based on a center frequency and a bandwidth. The “macrocell” is a cell of a base station that can perform transmission with high transmission power, that is, a base station having a large communication area. The “femtocell” is a cell of a base station that performs transmission with low transmission power, that is, a base station having a small communication area.
In recent years, communication utilizing a plurality of frequency bands has been investigated to widen the band. For example, regarding the 3GPP LTE-Advanced, a communication technology of carrier aggregation (hereinafter, referred to as “CA” in some cases) has been investigated. The CA is a communication technology utilizing a plurality of component carriers (hereinafter, referred to as “CC” in some cases). The CC means a unit of the frequency band that can be used for communication. In other words, the CA is a technology that can perform communication using different frequency bands at the same time. That is, the CA is a communication technology that can use a plurality of cells at the same time. The CA may be performed by a plurality of base stations cooperating with each other, or may be performed by using a plurality of cells formed by one base station.
To perform CA, first, a “primary cell (hereinafter, referred to as a “P cell” in some cases)” is set as a cell corresponding to a primary first CC. Then a “secondary cell (hereinafter, referred to as an “S cell” in some cases)” serving as a cell corresponding to a second CC that is different from the first CC is integrated into the P cell, and the P cell and the S cell are used at the same time to perform CA. Because a communication terminal can be connected to only one cell when a radio channel is set, the cell to be connected therewith when the radio channel is set is the P cell. Thereafter, the P cell is changed in handover and the like. Alternatively, the S cell can be added, eliminated, or changed. The P cell is used for both of a control channel and a data channel. In contrast, the S cell is used only for the data channel. Accordingly, for example, the data channels of both of the P cell and the S cell are scheduled using the control channel of the P cell. The S cell is added, eliminated, or changed also using the control channel of the P cell, for example.
In the related art, a technology has been developed for determining the maximum transmission power of a mobile station with respect to each CC to transmit various channels at the same time in a wireless communication system to which the CA is applied. Related-art examples are described, for example, in International Publication Patent No. WO 2010/103725 and Japanese Laid-open Patent Publication No. 2012-216969.
A wireless communication system to which the CA is applied (hereinafter, referred to as a “CA system” in some cases) can cause one communication terminal to use a plurality of cells at the same time. Among the communication terminals, it is appropriate for some communication terminals and not appropriate for some communication terminals to perform CA, depending on a status and the like of each communication terminal. For example, communication resources are wasted when the CA is performed by a communication terminal that is not required to perform CA because an amount of data to be transmitted is small. Accordingly, it is important in the CA system to efficiently perform communication by improving flexibility in changing a cell used by each communication terminal.